Tofisopam is 1-(3,4-dimethoxy-phenyl)-4-methyl-5-ethyl-7,8-dimethoxy-5H-2,3-benzodiazepine, which can be represented by the formula:

Tofisopam (racemic mixture) has been marketed under the names Grandaxin® and Seriel® as an anxiolytic. Although tofisopam is a benzodiazepine, it differs structurally from the classical diazepam-like benzodiazepines in that the nitrogen atoms in the ring structure are positioned at 2,3 instead of 1,4. Despite the structural similarity between tofisopam and classical 1,4-benzodiazepines, the difference in position of the nitrogen in the benzodiazepine ring confers pharmacological activity on tofisopam that is very different from classical benzodiazepines.
A synthesis of tofisopam is described in U.S. Pat. No. 3,736,315. Tofisopam has a chiral center at carbon C-5 and therefore has two enantiomers. In addition, each enantiomer of tofisopam can exist in two stable conformations based on the two configurations that can be assumed by the nitrogen containing benzodiazepine ring.
The molecular structure and conformational properties of tofisopam have been determined by NMR, CD and x-ray crystallographic methods (Visy, J. and Simongi, M., Chirality 1:271–275 (1989)). The 2,3 diazepine ring exists in two kinds of boat conformation. In the major conformers, (+)R and (−)S, the ethyl group attached to the center of asymmetry C-5 has a quasiequatorial orientation, while in the minor conformers, (−)R and (+)S, the ethyl group is positioned quasiaxially. Thus, racemic tofisopam can exist as four molecular species, i.e., two enantiomers each of which exists in two chiral conformations. The sign of optical rotation is reversed upon inversion of the diazepine ring. In crystal form, tofisopam exists only as the major conformations, with levorotatory tofisopam being of the (S) absolute configuration (Toth, G. et al., J. Heterocyclic Chem. 20:709–713 (1983); Fogassy, E. et al., In: Bio-Organic Heterocycles, Van der Plas, H. C., Ötvös, L., Simongi, M., eds. Budapest Amsterdam: Akademia; Kiado-Elsevier, 229:233 (1984)).
The absolute configuration of an asymmetric drug molecule can have profound effects on the efficacy of the drug. Fogassy et al., states that an abstract by Petocz et al. from a 1980 meeting describes pharmacological tests in mice which show different biological activity for the stereoisomers of tofisopam, including the observation that the activity of racemic tofisopam does not correspond with the sum of the activities of its enantiomers (Fogassy, E. et al., supra). However, Fogassy et al. does not describe the biological assays of the specific results achieved by Petocz et al. Furthermore, a search of the prior art yielded no such abstract by Petocz et al. Thus, there is currently no indication that Petocz et al. exists or relates to S-tofisopam and its unexpected properties.
In addition the binding of tofisopam enantiomers to human serum albumin has been reported to be stereoselective and affected by the interconversion of conformations (Simonyi, M., and Fitos, I., Biochem Pharmacology 32:1917–1920 (1983)).
Hungarian Patent No. 178516 describes an attempt to separate the enantiomers of tofisopam and observations relating to the administration of the separated products in mice. However, the purity of the separated products administered to the mice is not reported. Further, the absolute configuration of the separated products is not reported, and none of the tests in mice measured the anti-convulsant activity of the separated products.
There have been two reports that tofisopam exhibits anti-convulsant activity in mice. In 1981, C. Ito alleged that tofisopam can inhibit convulsions induced by tryptamine in mice (Ito, C., Tokyo Med. College 39:369–384 (1981); hereinafter “Ito”). However, the convulsion data of Ito does not support this conclusion. The administration of tofisopam according to the tests described in Ito appeared to have no effect on decreasing the incidence of convulsions in the mice (Table 6, Ito supra). Furthermore, Ito did not test the anti-convulsant activity of S-tofisopam substantially free of R-tofisopam.
In 1986, Pellow et al. reported that the administration of 100 mg/kg of tofisopam reduced the number of mice having convulsions induced by the compound Ro 5-4864 (Pellow, S. and File, S., Drug Dev. Res. 7:61–73 (1986)). However, Pellow et al. also reported that all of the treated mice still experienced myoclonic jerks. In contrast, Pellow et al. reported that 25–50 mg/kg tofisopam had proconvulsant activity in Tuck No. 1 mice when administered in combination with 3 mg/kg picrotoxin or 30 mg/kg pentylenetetrazole. Pellow et al. also reported that a dose of 10–50 mg/kg of tofisopam had no affect on the number or severity of convulsions in Tuck No. 1 mice that had been given 6 mg/kg of picrotoxin. Likewise, in Tuck No. 1 mice that were given 60 mg/kg of pentylenetetrazole, a dose of 10–25 mg/kg of tofisopam was reported to have no effect as an anti-convulsant. Pellow et al. did not test the anti-convulsant activity of S-tofisopam substantially free of R-tofisopam.
Numerous other reports, some of which were published after 1986, state that tofisopam has no anti-convulsant properties (Mennini et al., Arch. Pharmacol. 32:112–115 (1982); Saano, V., Med. Bio. 64:201–206 (1986); Petocz, L., Acta Pharm. Hung. 63:79–82 (1993); Szego, J. et al., Acta Pharm. Hung. 63:91–98 (1993)). None of the studies tested the anti-convulsant activity of S-tofisopam substantially free of its (R) enantiomer.
Tofisopam has been reported to enhance the actions of benzodiazepine anti-convulsants but not phenytoin, sodium valproate or carbamazepine (Saano, V., Med. Biol. 64:201–206 (1986)). For example, the potentiation action of tofisopam was reported to be effective with diazepam against convulsions (Briley, M. Br. J. Pharmacol. 82:300P (1984); Mennini, T., Naugn-Schmiedeberg's Arch Pharmacol. 321:112–115 (1982)), and against tremors (Saano, V., Pharmacol. Biochem. Behav. 17:367–369 (1982); Saano, V., Med. Biol. 61:49–53 (1983)). None of these potentiation studies examined the effects of either of the enantiomers of tofisopam on the anti-convulsant activity of diazepam or other anti-convulsants.